as a single unit, for example, a watershed or groundwater recharge zone. This chapter describes the conceptual and practical bases for landscape analysis of agricultural nonpoint source pollution and discusses options for and obstacles to implementing this approach.

NONPOINT SOURCE POLLUTANT ATTENUATION MECHANISMS

The basis for a landscape approach to agricultural nonpoint source pollutants is the use of particular areas as sinks for pollutants moving off agricultural fields. These sinks must be capable of intercepting the pollutants in either surface water runoffs and/or groundwater flows (Figure 12-1) and must support one or more of the processes that remove pollutants. These processes include plant and microbial uptake of nutrients and trace metals, microbial degradation of organic compounds, sediment trapping, microbial conversion of nitrate into nitrogen gas, and physical and chemical adsorption of metals and organic compounds.

Planning, implementation, and evaluation of the use of landscape sinks for nonpoint source pollutant control must consider two key factors: (1) the capability of a particular area to intercept surface water- and/or groundwater-borne pollutants and (2) the activities of different pollutant removal processes. Analysis of these factors is relevant in several contexts including field-scale development of specific off-field control practices such as grass vegetative filter strips, farm-scale analysis of where off-field controls should be established, and watershed-scale analysis of the effectiveness of existing sink areas such as riparian areas or wetlands.

Sediment Trapping

Extensive research has demonstrated that grass vegetative filter strips have high sediment trapping efficiencies if the flow is shallow and the vegetative filter strips are not filled with sediment. Trapping efficiency has been found to decrease dramatically at high runoff rates (Barfield et al., 1979; Schwer and Clausen, 1989). Several short-term experimental studies have reported the effectiveness of grass filter strips in reducing the amounts of sediments in runoffs (Dillaha et al., 1988; Magette et al., 1987; Young et al., 1980). These short-term studies found that grass filter strips are effective at removing sediments and sediment-bound pollutants at trapping efficiencies that exceed 50 percent if the flow is shallow (shallow flow refers to water flowing in sheets across a field or grass filter strip). Grass filter strip

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